1. Technical Field
The present disclosure relates to a hearing aid, a loudspeaker, and a feedback canceller that include configurations capable of suppressing occurrence of feedback.
2. Related Art
In general, a hearing aid includes a microphone for collecting sound transmitted from external space and a receiver for outputting the sound to the external ear canal of a user. Thus, feedback may occur when the output sound from the earphone is fed back to the microphone. Feedback cancellers are known as devices for suppressing occurrence of such feedback. Among them, a feedback canceller using an adaptive filter for adaptively estimating a feedback transfer function can be effectively installed in a hearing aid implementing digital signal processing.
Usually, when the adaptive speed of the adaptive filter is set at a high level, the transfer function converges quickly. However, this increases errors and makes entrainment prone to occur. The entrainment refers to a phenomenon that an input signal close to a sinusoidal wave is distorted due to malfunction of a feedback canceller using an adaptive filter in a hearing aid.
Therefore, it is not preferred that the adaptive speed of the adaptive filter be kept high.
On the other hand, when the adaptive speed of the adaptive filter is set low, the accuracy of estimation improves and entrainment is less prone to occur. In this case, however, it takes time to converge the transfer function. Thus, there have been suggested feedback cancellers that appropriately control the adaptive speed of the adaptive filter depending on situations (for example, refer to JP-A H6-189397 and JP-A 2007-515820).
For instance, in the feedback canceller included in the hearing aid disclosed in JP-A H6-189397, the adaptive filter operates at a low adaptive speed at normal times. Upon occurrence of feedback, the adaptive speed of the adaptive filter is switched to high by a manual switch.
In addition, for instance, in the feedback canceller included in the hearing aid disclosed in JP-A 2007-515820, the adaptive speed of the adaptive filter is changed according to the magnitude of a difference between an input signal and an error signal.